Wireless power transfer systems are loosely coupled magnetic systems which require an adequate field strength to be provided at a required distance from the power “transmission” structure to the power “receiving” structure. One example is charging electric vehicles. Power may be transmitted from a magnetic flux coupling device provided in or on a ground surface to a vehicle in the vicinity of the device. The ground device must have mechanical robustness coupled with the ability to ‘throw’ magnetic flux in a high arching pattern so that a large part of that flux can be collected by another (possibly similar) flux coupling device on the vehicle. This is not a trivial task made more difficult by the stringent conditions pertaining to electric vehicles. The apparatus may also need to be capable of allowing power transfer in the reverse direction, for example with bi-directional systems.
Flux coupling devices are referred to in this document as “pads” for convenience, although they may take other forms.
One known form of pad for electric vehicle (EV) charging applications is generally circular in shape, and is described in International patent publication WO2008/140333. An attractive feature of these pads is that they have low emissions. However, the useful flux that they can provide is relatively small, and the height, i.e. distance that they can provide flux beyond the pad, is likewise small. A generally circular pad as described in the publication referred to above which has a diameter D can achieve a useful flux height of perhaps D/4. Thus for an EV with a 200 mm air-gap a pad of 800 mm diameter is needed.
Alternatively, a magnetically polarised pad structure which has two coils or windings may be used. This general structure is described in International patent publications WO2010/090539 and WO2011/016737. Pads that incorporate this structure are referred to herein for convenience as a “DD” pad (with variants “DDQ”, and “Bipolar”). The DD type pad can throw flux much further—ideally twice as far as the circular pad. It is a polarized pad where a choice has to be made between laying the pad longitudinally along the direction of travel (XX), or transversely across the direction of travel (YY). The pads in the ground must have the same orientation as the pad under the EV or power transfer is not possible. The DD pad has two identical windings that touch (or almost touch) each other in the centre of the pad. These windings may be flat Archimedean spirals and they may sit on a bed of ferrite or striated ferrite strips. Characteristically the field is only out one side of the pad as the ferrite acts to concentrate flux behind the windings in the region between the pole areas so that there is no flux out the back of the bad. This is a highly desirable feature especially for the on-vehicle pad as it means that there is no flux in the cabin for the on-vehicle pad.
The flux pattern of the DD pad is characterised two flux areas. In the centre of the pad the flux paths are high and convex upwards and are ideal for linking to another DD pad. At each end of the pad the flux cannot link to another pad as it is turning away from any suitable pad. So these end fluxes produce no useful output. This wasted flux adds to the pad's inductance, reduce its coupling factor, and cause losses in any metal pieces it comes in contact with under the car. A high fraction of the total flux produced is in this category so if it could be reduced the pad would be improved.
Another pad structure comprises a simple solenoidal coil which is wound on a bar of ferrite. The pad produces flux out both sides and commonly one of these is removed using an aluminium screen. This removal is not very satisfactory and the pads have residual end flux that is very difficult to eliminate. For this reason these pads are invariably used in the direction of travel (XX) so that the distance to the edge of the car is greater and there is a longer section to remove the unwanted flux as cars are typically longer than they are wide. The solenoid pad can produce high useful flux patterns but they are less efficient as there are significant losses in the aluminium screens. However they do not have a high leakage and this is a beneficial feature.
The discussion above, and any references to prior art in this document, are not to be taken as an admission that the prior art referred to is publicly available in any country, nor that the prior art referred to is common general knowledge.